1. Field of the Invention
This present disclosure describes an active common mode EMI filter employing a number N of series cascaded active filter subcircuits. The power dissipation in the overall active filter, and the required voltage rating of each transistor, is reduced by 1/N, versus known single stage circuits. The power dissipation in each transistor is reduced by 1/N2.
The described technique is particularly advantageous for PWM inverter drives that operate from three-phase ac line voltage at 380-480V, where the power dissipated in a single stage active filter circuit may be unacceptably high.
2. Background Art
Previously disclosed active common mode EMI filter circuits have generally been based on a current amplifier with unity gain in a feedforward circuit. One previously disclosed circuit is shown in FIG. 1. This circuit employs IGBTs as the active elements. Other circuits employing MOSFETs and bipolar transistors have previously been disclosed, and follow the same operating principles.
In FIG. 1, the required filter bus voltage, Vbus filt, is determined by the product of the charge, Qtot, associated with each pulse of common mode current, and the value of the coupling capacitor CF CF is determined by the permissible amount of line frequency ground leakage current.
Vbus filt must be higher than Qtot/CF. Typically, the minimum required value of Vbus filt thus determined may be substantially less than the full dc bus voltage, Vbus drive, of the PWM inverter.
The average current Iav consumed by the transistors of the active filter is Qtotxc2x7f, where f is the inverter PWM switching frequency.
The single stage transistor amplifier in FIG. 1 could be connected directly across Vbus drive, even though this voltage might be substantially higher than the minimum needed by the active filter. The disadvantage would be that the voltage rating and power dissipation in the transistors would be higher than necessary. The total combined average power dissipation in the two transistors in this case would be Qtotxc2x7fxc2x7Vbus drive.
A previously described method for reducing the required voltage rating and power dissipation in the transistors of the active filter in the circuit of FIG. 1, is to derive a lower filter bus voltage from Vbus drive, via a voltage dropping resistor, as shown in FIG. 2. The required voltage rating of the transistors, and the total combined average power in the two transistors of the active filter, are reduced by Vbus filt/Vbus drive. The total combined power dissipation in the voltage dropping resistor, the two transistors of the active filter, and the voltage clamping zener diode, however, exceeds Qtotxc2x7fxc2x7Vbus drive. This is because the average current drawn through the dropping resistor from the dc bus is now higher than Qtotxc2x7f, due to the added current IZ drawn by the zener diode.
Another previously disclosed circuit is shown in FIG. 3. This circuit is similar to the circuit in FIG. 1, but employs N-channel MOSFETs as the active elements, and has a voltage divider R1, R2 for balancing the average voltages across Q1 and Q2. A difficulty with this circuit (as well as with the circuits of FIGS. 1 and 2) is that the MOSFETs (IGBTs) draw significant gate current from the overwindings on the current-sensing transformer, via the gate-source (gate-emitter) and drain-gate (collector-gate) capacitances. This causes a significant error in the output current of the amplifier.
To meet these needs, various embodiments of the invention provide an active EMI filter for reducing common mode noise current in a circuit comprising a rectifier coupled to an AC network, the rectifier supplying DC power to a DC bus, the DC bus feeding an inverter stage for providing AC power to a load, the load having a ground return line to a ground connection of the AC network. The active filter preferably comprises a transistor stage, a current sensor such as a current transformer coupled to a branch of the circuit having the common mode noise current flowing therein, the current sensor having an input such as a primary winding coupled for sensing said common mode noise current and an output such as a secondary winding driving the transistor stage, the transistor stage comprising two transistors driven by said output of the current sensor in response to the common mode noise current, and a capacitor coupling the transistor stage and the ground return line, the capacitor providing a cancellation current to the ground return line from the transistor stage to substantially cancel the common mode current in the ground return line.
The current sensor and the transistor stage are coupled in a feed forward arrangement whereby the transistor stage is coupled between the rectifier and the current sensor, the transistor stage and said current sensor having an amplitude gain of approximately unity.
In the transistor stage, a control electrode drive current may be provided to each transistor by a local power source such as a storage capacitor, via a drive transistor. There may be two drive transistors, each having two main electrodes, one main electrode of each drive transistor being connected to a respective control electrode of one of said two transistors, and each drive transistor having a control electrode, the control electrodes of the drive transistors being coupled respectively to additional secondaries of the current transformer.
The other main electrode of each of the drive transistors may be connected to the local power source, which may be a respective local storage capacitor which is connected across the corresponding drive transistor and its respective current transformer secondaries.
The local power source may further comprise two dropping resistors, connected in a series network alternating with the local storage capacitors. The network may be connected across a DC supply voltage. A voltage regulator, for example a zener diode, may be connected across each of said local storage capacitors.
Also, the filter may comprise a plurality of cascaded transistor stages coupled in series for providing an active common mode EMI filter with reduced power dissipation and transistor voltage rating.
Preferably there are a plurality of capacitors which respectively couple each of said transistor stages to said ground return line.
Each transistor stage preferably has a respective local current transformer, said local current transformer having a primary which is coupled to said output of the first-mentioned current transformer, and having a secondary which is coupled to the corresponding two transistors. A main electrode of each transistor may be coupled to a respective secondary of said local current transformer, and a control electrode of each of said transistors may be coupled to another respective secondary of said local current transformer. The other secondaries of said local current transformers may provide bias voltages for the control electrodes of said corresponding two transistors.
Preferably the primary and secondary of each said local current transformer have the same number of turns, while the other secondary of the local current transformer has a greater number of turns than the primary.
Advantageously the secondary of the main (first-mentioned) current transformer has N times the number of primary turns, N being the number of transistor switching stages.
In all embodiments of the invention, a respective local filter bus capacitor may be connected across each said transistor stage.
Other features and advantages of the invention will be appreciated from the following description of several embodiments thereof, with reference to the drawings, in which like references denote like elements and parts.